The invention involves a process for fixing a toner image transferred onto an image-carrier substrate, a process for fixing a single-color or multi-color toner image transferred onto an image-carrier substrate, a digital printer or copier machine that has a fixing device for fixing a toner image onto an image-carrier substrate, and a digital printer or copier machine, wherein at least two electromagnetic radiation pulses are applied in a time-delayed manner onto the same area of the image-carrier substrate.
A known process is electrostatic printing, in which a latent electrostatic image is developed by charged toner particles. These particles are transferred onto an image-carrier substrate, such as paper, for example, hereinafter referred to simply as xe2x80x9csubstratexe2x80x9d. Afterwards, the developed image that has been transferred onto the substrate is fixed by the toner particles being heated and fused, and possibly the substrate being heated. In order to fuse the toner particles, contacting processes are often used in which the toner particles are brought into contact with suitable devices, for example, hot rollers or cylinders. It is disadvantageous that the design, the maintenance and the operating costs of these heating devices that operate by contact are expensive and thus cost-intensive. In addition, it is usually necessary to use silicone oil as a separating agent that should prevent an adhesion of the fused toner onto the heating device. Furthermore, the defect rate, especially paper jams, caused by the contacting heating devices, is relatively high.
In order to fix the toner that is transferred onto the paper, for example, heating devices and processes are also known that operate in a non-contact manner, in which for example, the toner particles are fused, for example, using heat radiation and/or microwave radiation or with hot air, so that they adhere to the paper.
A known fixing device has a xenon lamp that is arranged above the transport path of the paper. Using the xenon lamp that is electrically powered by a power supply unit, a flash/radiation pulse or a continuous radiation can be applied onto the paper when the paper is guided past the xenon lamp. The toner image is fused, by the clocked or continuous electromagnetic radiation, and liquefies so that after it has cooled off, it adheres in a desirable manner to the paper surface. Xenon flash lamps emit electromagnetic radiation, mainly in the visible and near infrared wavelength range, in which the toner has a high absorption and the paper has only a low absorption. This known phenomenon leads to a non-uniform heating of the areas of the toner image, which have variably high toner densities. In the areas of the toner image with a low toner density, in which the toner particles are arranged more or less individually, the toner temperature is clearly lower than in the areas with a high toner density, because the areas with the high toner density absorb a large portion of the electromagnetic radiation. This different absorption behavior leads to a non-uniform fusing of the toner image in the areas with varying toner density. If the toner image is impinged with an energy that is so high that the toner is also fused in the areas with a low toner density, the so-called xe2x80x9cmicro-blisteringxe2x80x9d frequently occurs in the areas of the toner image with a high toner density, i.e. a bubble forms within the fused toner layer as a result of overheating of the toner and possibly the paper. It is disadvantageous in this that the gloss of the toner image is influenced in an undesirable manner. Furthermore, a partial overheating of the paper can occur, so that it begins to undulate.
Xenon flash lamps for fixing a single color (black) toner image, which emit electromagnetic radiation in the visible and short infrared range, have been known for a long time. The absorption capacity of the toner in the three process colors cyan, magenta, and yellow on the one side and the absorption capacity of black toners on the other side differ considerably in the wavelength range emitted from the xenon flash lamp. The process color-toner portions absorb only in a very narrow wavelength spectrum in the visible range and customarily absorb less than 10% in the near infrared range. Black toners absorb approximately 100% in the aforementioned wavelength ranges. These varying absorption characteristics lead to a non-uniform fusing of the toner image when the light of a xenon flash lamp is used to fix the toner image. A non-uniform fusing of the toner image leads to a non-uniform fixing of the toner, to a non-uniform gloss, to a partial bubble formation in the toner image or to a partial overheating and discoloration of the paper. This effect is especially yielded between the three process colors cyan, magenta, and yellow, which absorb the electromagnetic radiation emitted from the xenon flash lamp differently, but each selectively in a wavelength range between 0.25 xcexcm and 2 xcexcm, in particular in the range 0.4 xcexcm and 1 xcexcm. In this wavelength range, black toner absorbs approximately 100% of the electromagnetic radiation.
In order to match the absorption capacity of the process color toners to each other, an infrared absorber is added to them, for example, such that they obtain the same absorption characteristic as black toner in a wavelength range between 700 nm and 2 xcexcm. These types of absorbers, however, are not completely colorless in the visible range, so that they act in a disadvantageous way on the color reproduction. The better the absorption capacities of the process color toners are matched to each other using the infrared absorbers, the greater is their overlap with the visible range.
The purpose of the invention is to provide a process in which the toner to be fixed is fused using electromagnetic radiation, whereby the areas of the toner image with higher and with lower toner density have at least approximately the same fusing quality. Another purpose of the invention is eliminating defects in the toner image, which result due to a non-uniform energy absorption of the toner image. Another purpose is providing a process in which the process color toners impinged with electromagnetic radiation and the black toner have an improved uniformity in their absorption capacity. Finally, another purpose of the invention is to provide a digital printer or copier machine to perform the process.
In order to achieve the purpose, a process is characterized in that in order to fuse the toner particles at least two electromagnetic radiation pulses are applied onto the same area of the image substrate in a time-delayed manner. The second radiation pulse/flash is then triggered, for example, when the intensity of the first radiation pulse/flash has been reduced to a certain value. The term xe2x80x9ctime-delayedxe2x80x9d is understood here to be the time duration between the triggering of the first radiation pulse/flash and the triggering of the second radiation pulse/flash. It has been revealed that by the time-delayed application of the second radiation pulse, the limit value of the energy, at which the toner image is overheated, increases. According to the invention, it is thus possible that to fuse areas of the toner image with high and low toner density, the same energy can be applied to each, without a bubble formation occurring in the fused toner layer in areas with high toner density. The energy of each individual radiation pulse is in each case below the limit energy at which a bubble formation would occur in the molten mass in the areas of the toner image with high toner density. The total of the energy of all radiation pulses is in any case so high that even areas of the toner image with low toner density are fused in the desired way and in this way fixed onto the image-carrier substrate. With process according to the invention, an at least approximately equivalent fusing quality of the areas of the toner image with high and with low toner density can thus be ensured. In addition, it is advantageous that damage to the toner image and the image-carrier substrate as a result of excessive heating is avoided.
In the following, a brief description is given of what the term xe2x80x9ctoner densityxe2x80x9d means in connection with the invention presented here: In a color print, the toner image can have four different colored toner layers, for example, whereby usually one of the toner layers is black, yellow, magenta, or cyan. The maximum density of each toner layer on the image-carrier substrate is 100% corresponding to a density measured in transmission of approximately 1.5, whereby a maximum total density of the toner layers/toner image of 400% is produced. Usually the density of the toner image is in a range from 10% to 400%. A toner image with only a 10% density is mainly formed by individual toner particles on the image-carrier substrate. The energy required to fuse a toner image with a toner density of 10% is distinctly higher than the energy that is required to fuse a toner image with a toner density of 400%.
In a preferred embodiment form, the total radiation energy density of the at least two radiation pulses, which is required to fuse the toner in the desired manner, is equally as large at very low toner densities, i.e. 10% for example, and at high toner densities, i.e. 290% or more. Since a toner image usually has areas with high and with low toner densities, it can be ensured that none of these areas, especially also those with a high toner density, are overheated and that the entire toner image is fused uniformly.
The principle of the aforementioned process is characterized in that the maximum radiation energy of each radiation pulse is less than the limit energy density, at which bubble formation would begin when it is transmitted onto the toner image having a toner layer with a high toner density and/or having the highest toner density. The level of the radiation energy density of at least two radiation pulses is, however, sufficiently high so that after the last of the radiation pulses has been applied onto the toner image and/or onto the area to be fixed, the radiation energy density required for fusing of the toner area was transferred onto it.
An embodiment example of the process is preferred in which the total radiation energy density of the at least two radiation pulses is in a range from 1 J/cm2 to 18 J/cm2, preferably from 3 J/cm2 to 10 J/cm2. It has been revealed that with this total radiation energy density a wide toner density range can be covered.
In a preferred embodiment, the radiation energy density of an individual radiation pulse is in a range from 0.5 J/cm2 to 5 J/cm2. The respective radiation density of the individual radiation pulses can thus be distinctly less than the required total radiation energy density that is required to fuse the toner layers with only a low toner density.
Finally, an embodiment example of the process is preferred that is characterized in that the time interval between two subsequent radiation pulses is approximately 10 ms to 1000 ms. Preferably, the time interval is selected depending on the respective radiation energy density of the radiation pulse and the required total radiation energy density that must be introduced into the toner image for its uniform fusing.
It is readily apparent from the above that in order to fuse the toner particles of the toner image transferred onto the image-carrier substrate, more than two electromagnetic radiation pulses, for example, 3, 4, or 5 radiation pulses, can be applied onto the fixing area of the image-carrier substrate in a time-delayed manner. The higher the number of the radiation pulses is, the smaller the radiation energy density of each individual one of the radiation pulses can be. Furthermore, the time interval between every two subsequent radiation pulses and the intensity and length of the individual pulses can also be varied. It is important that even areas of the toner image at low toner density are fused in a desired manner, and that in the process, the areas of the toner image with high toner density are not overheated causing bubbles to form in the molten mass.
In order to achieve the purpose of the invention, a process is also proposed that functions for the fixing of a single or multicolor toner image whereby to fuse the toner image it is impinged with electromagnetic radiation. The process is characterized in that the toner image is predominantly impinged with electromagnetic radiation in the UV range (ultraviolet range). The wavelength range of the UV radiation is in a range from 200 nm to 380 nm. It has been revealed that within this wavelength range, the absorption capacity of the toner with the colors cyan, magenta, and yellow, hereinafter referred to simply as xe2x80x9cprocess color tonersxe2x80x9d, and black are similar to each other, since the absorption is done predominantly through the toner resin. Since the multi-color toner image is only impinged with the UV range of electromagnetic radiation, a uniform fusing and fixing of the different toners are ensured. In this way, a uniform gloss can be achieved over the entire toner image.
According to an additional embodiment of the invention, it is provided that the electromagnetic radiation is emitted by at least one flash lamp, and that except for the UV portion of the radiation, the remaining spectral range of electromagnetic radiation is filtered out before the radiation hits the toner that is to be fixed. The fixing range of the toner image is thus impinged with timed electromagnetic radiation in the UV range. Since the undesired wave range of the radiation emitted by the flash lamp is filtered out, practically any radiation source can be used, for example, a xenon lamp.
An embodiment example is especially preferred in which at least one radiation pulse emitted by the flash lamp has a high UV-portion in relation to the total radiation. This can, for example, be ensured with a xenon/mercury lamp that, after reaching its operating temperature, which is above the boiling point of mercury, emits an electromagnetic radiation that has a clearly higher UV-portion compared to a conventional xenon lamp.
In a preferred embodiment of the invention, it is provided that at least two short radiation pulses each having a high UV-portion are applied with a very small time delay onto the toner to be fixed. The radiation pulses/flashes are thus triggered such a short time after each other that they overlap each other, resulting in a radiation pulse that is almost longer. For example, a first lamp can emit a short radiation pulse, whereby a second lamp then only emits a radiation pulse if the power of the first radiation pulse has fallen below a certain limit value. Then, a third radiation pulse can be emitted if in turn the power of the second radiation pulse falls below a certain limit value. Provided additional radiation pulses are applied onto the fixing area, they can be correspondingly triggered in the manner mentioned above, i.e. with the corresponding time interval between two radiation pulses that follow each other. When the individual pulses are shortened, the color-dependent fixing UV-portion increases.
The fixing conditions are preferably adjusted to the toner of the toner image, which has the lowest absorption capacity of the UV radiation. If the toner image has, for example, a yellow toner layer, then during continuous electromagnetic radiation, its time duration and/or the level of its energy density are adapted to it, and during a clock-pulsed electromagnetic radiation, the number of the radiation pulses applied to the fixing area, their respective energy density and/or time interval between two successive radiation pulses and the like, are adapted to it. This means the fixing conditions are tuned such that on the one hand, even a yellow toner is fused in the desired manner, and on the other hand, an overheating of the image-carrier substrate and the remaining color toners is prevented with certainty.
Finally, an embodiment example of the process is preferred in which to adjust its different absorption capacity of electromagnetic radiation, the respective fusing properties of the different-colored toners are optimized depending on the respective toner color so that the color-dependent differences in the energy absorption are equilibrated. This can be done, for example, by modification of the molecular weight distribution or the glass transformation point or by different mixture ratios of two or more polymers or by the addition of different concentrations of other additives that influence the fusing behavior, such as for example, waxes. In this way, a uniform fusing of the different colored toners is achieved. Furthermore, damages in the toner image, for example, fusing explosions, can be prevented with certainty.
In order to solve the purpose of the invention, a digital printer and copier machine proposed which includes a fixing device with at least one radiation source, by which clocked electromagnetic radiation, i.e. radiation pulses, can be applied onto the image-carrier substrate. The machine has, furthermore, at least one power supply unit for the radiation source. The radiation source is, for example, made of a xenon lamp or a xenon/mercury lamp. The machine is characterized in that using the radiation source at least two time-delayed radiation pulses can be applied on the same area of the image-carrier substrate. The time interval between two successive radiation pulses can preferably be varied. Furthermore, the energy density of the respective radiation pulses can be adapted to the toner that is to be fixed on the image-carrier substrate. According to the invention, the fixing area of the toner image is thus irradiated with several radiation pulses so that their emitted total radiation energy density is sufficiently high to uniformly fuse and fix the toner areas with low and high toner densities.
In order to solve the purpose of the invention, a digital printer and copier machine is proposed which includes a fixing device with at least one radiation source, for example a flash lamp, for applying clocked electromagnetic onto the image-carrier substrate. The machine is characterized in that the radiation source is a xenon/mercury (Xe/Hg) lamp. The Xe/Hg lamp has several temperature-dependent operating states. A first operating state is present if the temperature of the Xe/Hg lamp is still below the boiling point of mercury. In this operating state, the Xe/Hg lamp acts like a normal xenon/mercury lamp with corresponding UV-radiation portion. A second operating state of the Xe/Hg lamp is achieved after it has a temperature that is above the boiling point of mercury, and the mercury is thus evaporated. In this operating state, the Xe/Hg lamp emits a considerable portion of its radiation flow in the UV-range. The machine according to the invention can be used in an especially advantageous way for the fixing of color toner images.
In an especially advantageous embodiment example of the machine, at least one filter is allocated to the radiation path of the Xe/Hg lamp and the image-carrier substrate, which lets only the UV portion of the electromagnetic radiation through. In this way, for process color toners, a uniform fusing and fixing of the toners onto the image-carrier substrate can be ensured because of their relatively equivalent absorption capacity in the UV-range, even without special absorbers having to be added to the toners for this purpose.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.